A high-power, high-repetition-rate THz source for pump–probe experiments at Linac Coherent Light Source II

Abstract

<jats:p>Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (∼10%), carrier-envelope-phase-stable THz pulses that produce high fields (&gt;1 MV cm<jats:sup>−1</jats:sup>) at the repetition rate of the X-rays and are well synchronized with them. In this paper, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.</jats:p>

A Compact Accelerator-Based Light Source for High-Power, Full-Bandwidth Tunable Coherent THz Generation

“…The electron accelerators have reliable abilities to produce THz radiation via several different ways: coherent synchrotron radiation (CSR) [18][19][20][21], optical transition radiation (OTR), coherent optical transition radiation (COTR) [22][23][24][25], or undulator radiation [26][27][28].…”

“…Generation of tunable coherent THz radiation with a pre-bunched electron beam has also been demonstrated in the storage ring, but with a relatively low power and long pulse length [20]. The CSR radiation can also be achieved with the linac accelerator where one single pass radiation will limit the radiation at low gain level and the output bandwidth is quite broad [21]. The OTR emits when the relativistic electron beam crosses the boundary of two different medias, which can generate the radiation from X-ray to microwaves, while the radiation generally has a broadband spectrum and a maximum pulse energy of hundreds µJ (350 µJ [24] and 140 µJ [25]).…”

“…Free electron laser (FEL) [30][31][32] is recognized as a new generation light source, which has the capability of providing ultra-short pulses with gigawatt (GW) level peak power and tunable wavelength from THz to hard X-ray. Most of the FEL facilities adopt afterburners to generate intense THz radiation by using a compressed electron bunch with the duration shorter than one THz period; therefore, no high gain process is expected and the radiation power is usually limited to a maximal pulse energy of hundreds µJ (200 µJ [21] and 100 µJ [33]). Powerful narrow-band THz pulses can be produced by using a sub-picosecond (ps) scale pulse train, which can be generated either by directly illuminating the photocathode with a train of laser pulses [34][35][36] or manipulating the electron beam at relativistic energy (including exchanging transverse modulation to longitudinal distribution [37,38], direct modulating the drive laser [39], converting wakefield induced energy modulation to density bunching [40], two wavelengths of energy modulation in two separated undulator sections [41,42], slice energy spread modulation [43], and laser-based density modulation [44,45]).…”

Field Analysis for a Highly-Overmoded Iris Line, with Application to THz Radiation Transport at LCLS

“…On the other hand, being larger than optical wavelengths makes the THz wave relatively more prone to Fresnel diffraction. An example of this challenge, which was the original motivator behind the present study, is the problem of efficiently transporting radiation from an "afterburner" THz linear undulator downstream of LCLS over a distance of 150-350 m, to reach the experimental halls at the LCLS facility, SLAC, Stanford [1,2]. A traditional quasi-optical solution that utilizes a combination of planar, toroidal or paraboloidal mirrors to relay the THz beam in multiple steps is one proposed solution, which typically suffers from power loss of approximately 1% per mirror as well as some aberration and misalignment [1,3].…”

“…An example of this challenge, which was the original motivator behind the present study, is the problem of efficiently transporting radiation from an "afterburner" THz linear undulator downstream of LCLS over a distance of 150-350 m, to reach the experimental halls at the LCLS facility, SLAC, Stanford [1,2]. A traditional quasi-optical solution that utilizes a combination of planar, toroidal or paraboloidal mirrors to relay the THz beam in multiple steps is one proposed solution, which typically suffers from power loss of approximately 1% per mirror as well as some aberration and misalignment [1,3]. To reach the near experimental hall at LCLS, for example, through a 150-m path (roughly 34 mirrors) going through the access maze at LCLS, the mirrors are estimated to incur around 30% power loss [1].…”